Several years ago, we discovered that fetal liver stem/progenitor cells (FLSPC), which have much higher proliferative activity than adult hepatocytes, replace 20-25% of liver mass through cell competition. Since transplanted adult hepatocytes do not have a proliferative advantage over host hepatocytes, they do not induce cell competition and do not repopulate the liver, unless there is extensive damage to the host liver to which the transplanted hepatocytes are resistant. However, major limitations in using FLSPC clinically for liver repopulation are logistic problems in obtaining sufficient cells for effective therapy and significant ethical concerns. We recently discovered that FLSPC hyperexpress the proliferative gene Yap, the effector of the Hippo signaling pathway that controls liver size, and survivin, a downstream target of Yap that inhibits apoptosis, a critical component in cell competition. We, therefore, hypothesize that there is cooperation between genes involved in cell competition and the Hippo signaling pathway and that if we introduce Yap into adult hepatocytes, we will impart in them the unique (proliferative and anti-apoptotic) properties of FLSCP that will allow adult hepatocytes to repopulate the normal adult liver. Since there are oncogenicity issues when introducing a proliferative gene, such as Yap, into cells, we have linked the estrogen receptor (ERT2) to Yap, so that tamoxifen can be used to control Yap's nuclear function. A major requirement for effective cell therapy is to maintain the cells in the host long-term and this has been problematic for transplanted hepatocytes in both animal models and in humans. Lenti-viral vectors are designed for permanent incorporation of transgenes into the cellular genome and we will test for durability of lenti Yap-ERT2 transduced hepatocyte transplantation in the Gunn rat hyperbilirubinemia model for Crigler-Najjar Syndrome, type 1 and whether tamoxifen retreatment long after initial therapy has been completed will reactivate transplanted cell proliferation and produce a therapeutic response resulting in decreased serum bilirubin. The Specific Aims are: 1) to repopulate the rat liver with lentivirus Yap ERT2 transduced hepatocytes in which the proliferative function of Yap is regulated by tamoxifen administration, determine the cellular and molecular mechanism(s) by which repopulation occurs and evaluate tumorigenesis in the basic repopulation model, 2) to assess tumor risk by identifying the genetic factors, steps and/or tissue derangements necessary to induce tumorigenesis by Yap transduced hepatocytes; and 3) to use our lenti-viral vector system to treat hyperbilirubinemia in the Gunn rat, to demonstrate that human hepatocytes transduced with lenti-Yap ERT2 can repopulate the mouse liver in an immunotolerant model that accepts human xenografts and to accelerate liver repopulation by WT hepatocytes in PiZ mice in the absence of AdHGF. By combining the proliferate power of Yap with the ability to control cell expansion by regulating Yap expression/function, this project represents a new approach to hepatocyte transplantation as a potential method to treat genetic-based disorders in liver metabolism.